It is known that in the processes of this type, so-called "PSA" (Pressure Swing Adsorption), there is progressively formed, in operation, a temperature gradient in the separation adsorbent bed, which results particularly, at the end of about ten hours of operation, in the creation of a particularly cold zone in the upstream portion of the separation bed, which greatly affects the production output during cyclic operation under vacuum or pressure, and this the more so as the operating pressure is higher (greater than 5.times.10.sup.5 Pa). To overcome this drawback, it has been proposed to heat the gaseous mixture to be separated, as described in U.S. Pat. No. 3,973,931 or to immerse in the upstream portion of the adsorbent bed a heat source, as described in the above-mentioned patent, or, more recently, in EP-A-502.627 which describes also an adsorbent unit of the type defined in the first paragraph. The heating of the gaseous mixture gives satisfactory results for very small installations but becomes inoperative, and moreover prohibitive, for installations with industrial capacity (greater than 0.5 ton of production gas per day). The inclusion in the adsorbent bed of effective heating means considerably increases the cost of production.